An adhesive is any substance applied to the surfaces of materials that binds them together and resists separation. Adhesives offer many advantages over binding techniques such as sewing, mechanical fastening, or thermal bonding. These include the ability to bind different materials together, to distribute stress more efficiently across the joint, increased design flexibility, and cost effectiveness.
Surface bonding techniques are preferred to mechanical fasteners primarily due to their superior load-transfer characteristics. For high-performance engineering applications, surface bonding is typically achieved using elevated temperature-cure, thermosetting adhesives. These thermosetting adhesives usually require temperatures of 120-200° C. for 5-120 min to complete the bond. The most common ways of heating adhesive bonds are convection ovens, thermal blankets, and radiant heaters.
An ideal adhesive is one that has long pot life, but can be cured immediately when needed. It is also highly desirable for the adhesive to cure without having to submit the entire part assembly to a large oven, thermal blanket, or radiant heater. Furthermore, for energy efficiency, one may also take advantage of the latent reaction heat from the adhesive system to sustain its own curing. Such an adhesive provides tremendous advantage for flexible and efficient manufacturing processes.
Frontal polymerization offers attributes that meet the above requirements. Frontal polymerization is a localized reaction that propagates through the coupling of thermal diffusion and the Arrhenius dependence of an exothermic polymerization reaction. The result is a localized thermal reaction zone that then propagates through the reactants as a thermal wave. Frontal polymerization exploits heat production because of exothermicity of the polymerization reaction itself and its dispersion by thermal conduction. If the amount of dissipated heat is not too great, then a sufficient quantity of energy able to induce the polymerization of the monomer close to the hot zone is provided. The result is the formation of a hot polymerization front capable of self-sustaining and propagating throughout the reactor.
Currently known frontal polymerization can only be triggered by direct (physical) contact to thermal (heat) sources or line-of-sight exposure to UV light. Yet, practical adhesives are often used to bond optically opaque parts for which neither triggered mechanisms is applicable.
Conventional approaches require either direct contact with a heat source (e.g., soldering iron) or line-of-sight exposure (UV light). Most adhesive bonding processes occur in between two optically opaque substrates; thus the approaches in the known art are not applicable. A solution to this problem is needed.